Many attempts have been made to design and build "stick" woodburning stoves as sources of either auxiliary or primary heat for residences and other structures. Such stoves have generally been inefficient, not completely burning the wood used for fuel. Large amounts of ash, unburned fuel and soot were left after the combustion process was completed. This was in part due to the inefficient design of the stoves themselves, and in part due to the varying size, shape and composition of the wood fuels which were used.
In an attempt to increase efficiency, stoves have been designed to burn "pelletized" fuel, which can be made from a wide variety of organic material. Such pelletized bio-mass fuels generally have a uniform or near uniform size, shape, moisture content, and composition. With the pellets having such consistency, the combustion of individual pellets is also uniform or near uniform, since the individual pellets of fuel have essentially the same surface area and composition.
In addition to the uniformity in size, shape and composition of fuel used in pellet stoves, another factor which greatly influences the efficiency of the combustion process in these stoves is the flow of combustion air into and around the area in which the fuel is burned.
"Pellet" stoves have been designed with control of the air flow through the combustion and exhaust system. One such type blows air into the combustion chamber by forcing the air at a pressure somewhat higher than the ambient air pressure into the combustion chamber. This is commonly known as "forced draft". This type of stove can have severe leaks of smoke and fumes into the area surrounding the stove if a leak develops anywhere in the entire system. This method also limits heat transfer ability due to the problems inherent in forcing exhaust gases through restricted heat exchangers.
Still another method which has been utilized is shown in U.S. Pat. No. 4,565,184 to Collins, et al. The Collins patent uses an exhaust fan to: (1) draw air through an input port or ports into the combustion chamber and; (2) out of the stove through the heat exchanger by decreasing the air pressure at the exhaust port below that of the ambient air pressure. This is commonly know as "induced draft".
This exhaust type of design is susceptible to air leaking "into" the stove which greatly reduce its efficiency. The entire system must be kept completely airtight. In addition, soot and fly ash are common residues of pellet stoves. As these byproducts collect in the heat exchanger and blower assembly, efficiency is greatly reduced. This is due to the rapid decline in the blower performance as air flow restrictions increase.